Bentonite is not only some type of material excellent in water absorption performance but also has the functional effect of permitting a decrease in water permeability in such a manner that the bentonite gets swollen to cause water-permeable voids to be decreased. Besides, the bentonite belongs in natural clay minerals and is thus considered to be a stable material ensuring that material performances are maintainable for long periods. For that reason, the bentonite has been heretofore employed in various fields, that is, as liquid stabilizers for use in excavating works, grouting materials, cement slurry additives or like materials.
In recent years, in situations where industrial and/or radioactive waste disposals become an issue, considerable advances have been made in examinations on use of the bentonite as the material of water shields and/or fillers in the industrial or radioactive waste final disposal facilities. For instance, in Patent document 1, there is disclosed a water shield in which coal ash and bentonite are contained. Alternatively, in Patent document 2, there is disclosed a radioactive waste burial filler comprising bentonite, volcanic glass and fly ash.
There have been also other examinations on application of the bentonite to engineered multi-barriers in the radioactive waste disposal facilities for very long-term underground burial of the radioactive wastes. Each engineered multi-barrier of this type is arranged such that a low water-permeability layer or a buffering material mainly comprising high-swelling sodium type bentonite is adjacent to a cement-based material used as a radionuclide low-diffusion layer, a timbering or like material. With such engineered multi-barriers, however, the possibility exists that there would be the occurrence of leaching-out of calcium ions from the cement-based material due to very long-term contact with the underground water around the disposal facilities, resulting in the problem about bentonite alteration such as bentonite dissolution and conversion of sodium type bentonite to the calcium type by the action of the underground water containing the leached-out calcium ions, together with the problem about a bentonite swelling decrease attendant upon such bentonite alteration. Thus, in this connection, some energetic examinations are now in progress to overcome the above problems.
For instance, in Patent document 3, there is disclosed a repository structure in which a buffering layer consisting of a silica-based material is interposed in a boundary between a water-shielding layer supposed to be formed with a bentonite-based material layer and a structure material made up of a cement-based material layer including a mortar or concrete layer with hydraulic cement as a binder in order to prevent the penetration of calcium ions from the above cement-based material layer into the bentonite-based material layer in the manner of causing calcium silicate to be produced in the buffering layer. Alternatively, in Patent document 4, there is disclosed a low water-permeability layer material obtained by adding sodium bicarbonate or sodium carbonate to sodium type bentonite.